1. Field of the Disclosure
The disclosure relates to systems and methods for inspecting lightning protection systems and, more particularly, to systems and methods for inspecting lighting protection systems prior to the fastener installation process.
2. Description of the Related Art
Today's aircraft are being designed and built with greater percentages of composite material. For example, the Boeing 787 “Dreamliner” aircraft has more than 50% composites for its primary structure. Although composites are lighter and have better mechanical and fatigue properties than traditional aluminum, they are less electrically conductive, and have poor electromagnetic shielding, resulting in poor current dissipation when lightning strikes the aircraft. Compared to traditional aluminum, composites, in some circumstances, may be subject to greater damage due to lightning strikes.
When lightning hits an aircraft, a conductive path on its skin allows the electricity to travel along the skin, and then exit at some other location. Without an adequate conductive path, arcing and hot spots may occur. Arcing and hot spots have the potential to char, delaminate and/or penetrate the skin. In some circumstances, the charring, delamination, or skin penetration may reduce the load-bearing characteristics of the structure. The low electrical shielding capability of composite materials may increase the likelihood that electrical circuits will be affected by the lightning strike.
One way to protect composite skins from lightning strike damage is to include conductive Lightning Strike Protection (LSP) systems either in or on the composite skins of an aircraft. An example of an LSP system is shown in FIG. 1. The LSP system can consist of conductive (e.g., copper) sheets or foils 110 on the layer of composite material 120 nearest the aero surface. Fasteners 140 in aircraft structures require that a countersunk hole 130 be drilled into the composite surface. Current practice is to position the fastener head 142 deeper into its countersunk area to ensure that a minimum distance can be established between the conductive foil 110 and the fastener head 142.
This Lightning Strike Protection system functions by diverting the electrical energy through the conductive foil 110. The conductive foil (nearer the surface) attracts lightning away from the fastener heads 142 which are slightly lower. Maintaining the set minimum distance between the conductive foil 110 and the fastener head 142 reduces the chance of arcing. Dielectric sealant may also be placed on top of the fastener head 142 to prevent arcing from the foil 110 to the fastener 140.
As illustrated in FIGS. 2A and 2B (The cross section of FIG. 2B is perpendicular to the cross section of FIG. 2A) and shown in FIG. 3, wrinkles in the conductive foil 110 may occur due to thermal conditions during the composite curing process. When heated, conductive foils 110 tend to expand more than the composite materials, due to differences in coefficient of thermal expansion (CTE). For example, copper expands much more than graphite fiber. When heated to the standard cure temperature of 350° F., the conductive foil 110 expands relative to the composite materials 120. Since the foil 110 is restrained lengthwise, wrinkling can result. These wrinkles are then locked in place upon curing of the composites, and, when cooled, the wrinkles remain. These wrinkles cause the conductive foil to extend in depth below the designed level. In the wrinkles illustrated in FIGS. 2A and 2B the wrinkle depth is below the top of the fastener head 142.
These wrinkles increase the likelihood of arcing occurring between the fastener 142 and the conductive foil 110 when the aircraft is in service. When a fastener hole 130 is drilled into a composite panel where a wrinkle in the LSP exists, the resulting hole and LSP may require repair or some other action to maintain the minimum distance between the LSP and the fastener. The repair may result in substantial expense and an increase in production time.
Accordingly, there is a need for a system and method for inspecting the lightning protection system prior to fastener installation to ensure that there will be minimum gap between the conductive foil and the fastener head when the fastener is installed.